Fourth-Generation Long Term Evolution (LTE) downlink signals are modulated using orthogonal-frequency domain multiplexing (OFDM). The LTE uplink, on the other hand, uses single-carrier frequency domain multiple access (SC-FDMA). SC-FDMA was selected for the LTE uplink because it has a much lower peak-to-average than OFDM, and therefore, it helps reduce the power consumption of mobile phones. However, the use of SC-FDMA also introduces problems. SC-FDMA is very susceptible to interference. Even a small amount of interference can degrade the performance of an entire LTE cell.
A leading source of interference impacting LTE networks is passive intermodulation (PIM) interference. PIM occurs when RF energy from two or more transmitters is non-linearly mixed in a passive circuit, such as bad RF connections, damaged cables, poor antennas or reflections from objects such as buildings. PIM becomes interference to a cellular base-station when one of the intermodulation products interferes with base-station receive channels (LTE uplink) that are utilized by mobile or stationary communication devices.
Interference is quite common in today's LTE network. Most roof-tops and cell towers are shared by many base-stations (co-located). With the increase in the number of transmitters at a given location, there is also an increase in the likelihood of generating an intermodulation product lands in one of the receive channels. In most communication environments involving short range or long range wireless communications, interference from unexpected wireless sources can impact the performance of a communication system leading to lower throughput, dropped calls, reduced bandwidth which can cause traffic congestion, or other adverse effects, which are undesirable.